A molecular beam study of the kinetics of the catalytic reduction of NO byCO on Rh(111) single-crystal surfaces

Steady-state rates for the catalytic reaction of NO with CO on Rh(111)surfa ces have been measured under isothermal conditions by using a molecular bea m approach with mass spectrometry detection. Systematic studies were carrie d out as a function of surface temperature, NO + CO beam composition, and t otal beam flux. A maximum in reaction rate was observed between 450 and 900 It, the exact temperature depending on the NO:CO beam ratio. Indeed, a syn ergistic behavior was seen where the loss in reactivity induced by increasi ng the CO concentration in the beam is partly compensated by a higher surfa ce temperature. The data presented here are consistent with the rate-limiti ng step of the overall NO reduction process being the surface recombination of atomic nitrogen atoms resulting from fast dissociation of NO adsorbed m olecules, Temperature-programmed desorption and CO titration experiments we re also performed after the isothermal kinetic runs in order to estimate th e surface coverages of the reactants during the steady-state reactions. The NO + CO conversion rate was found to be directly proportional to the cover age of atomic oxygen on the surface, The relation between reaction rates an d nitrogen coverages, however proved to be much more complex: an inverse co rrelation was in fact seen in most cases between those two parameters. The build-up of a critical coverage of atomic nitrogen was found to be necessar y to trigger the nitrogen recombination step to N-2. This critical coverage of strongly held nitrogen was determined to not depend in any significant way on the composition of the beam, but to decrease with reaction temperatu re in all cases. (C) 1999 Academic Press.